The invention relates to a flow-control valve for liquids, primarily liquids with temperatures in the vicinity of their boiling points, particularly for water heaters, consisting of a seal cooperating with a valve seat, which with the valve seat forms a throttle point.
Control valves which control the quantity of a given throughput are known. Valves of this type are also sometimes used to control liquids whose temperatures are in the vicinity of their boiling points. It is known, for example, that in water heaters, in particular coffee markers, valves must be installed through which boiling water flows. The problem then arises that depositions of lime must be prevented in order to maintain the function of the valve itself and hence the tight seal. Valves have been proposed for this purpose which have rubber membranes as seals, and these have the advantage that they are set vibrating every time they seal thus clearing away any lime automatically.
However, deposits of lime also have the disadvantage of altering the throughput by increasing the flow resistance. It has been shown, however, when the flow of boiling water is controlled, that at the narrowest flow cross section, which is generally the throttle point between the valve seat and the valve body, after the point where the flow passes over an edge or encounters a resistance, negative pressure occurs resulting in cavitation which can sometimes lead to considerable distortion of the flow. At the same time, at the points where this cavitation occurs, the undesirable deposition of lime occurs relatively rapidly and this in its turn encourages cavitation. Thus a purpose of the present invention is to design a flow-control valve of the type indicated at the outset which will prevent the occurrence of cavitation at the throttle point and hence also prevent lime deposition.
The invention contemplates providing ventilating holes at the throttle point downstream of the narrowest flow cross section. This arrangement counteracts negative pressure formation in the region of the throttle point because the ventilating holes naturally equalize the pressure. Uncontrolled formation of steam bubbles with their effect on the flow can be avoided.
It is advantageous for the ventilating holes to be shaped into an annular slit surrounding the throttle point with the ambient pressure acting on the slit via an annular chamber. Also it is structurally very simple to place the annular slit and the annular chamber between one of the inlet nozzles on which the valve seat is mounted and an inner tube inserted into the latter. In this way difficulties of production technology are avoided.
A particularly advantageous embodiment arises when the inlet nozzle flares conically outward away from the valve seat, because in this case the narrowest cross section is ahead of a flow cross section which can be kept substantially larger. In this way, the upper edge of the inlet nozzle can be formed directly as a valve seat and the end of the inner tube adjacent to the valve seat can be tapered conically upward at an angle which is as large as that of the inlet nozzle. This produces a nozzle-shaped structure at the valve seat through which the flow passes in the opposite direction. This has the great advantage that an annular zone can form downstream of the narrowest cross section which serves to equalize the pressure and prevent cavitation and in which any lime deposited at the edge has no effect on the magnitude of flow. There is an advantageous secondary effect that the equalization air or the colder air column flowing into the annular chamber in the opposite direction to the hot water can cause the inner tube to cool down relatively rapidly after the device is shut off, and this cooling action, if the material is suitably chosen, causes shape changes in the very thin inner tube caused by temperature fluctuations, and any lime deposits can easily flake away. A further advantage is that after the valve seat has closed, outflow from the inner tube is facilitated because the air does not have to flow from the bottom to the top through the liquid which is flowing downward, but can penetrate through the annular slit beforehand from the top. Hence the new valve allows the water to flow away from more rapidly than known valve designs. The inner wall of the conical part of the inner tube advantageously has a diameter somewhat greater than the diameter of the narrowest throttle cross section at the valve seat of the inlet nozzle, since it has been shown that direct impact of the stream of fluid flowing in through the valve against the upper edge of the inner tube and penetration of the fluid into the annular chamber is avoided.
These and further objects, features and advantages of the present invention will become more obvious from the following description when taken in connection with the accompanying drawings which show, for purposes of illustration only, a single embodiment in accordance with the present invention.